Here is the easiest way to earn this - me and a friend just earnt this each in a total of 10 mins.
To get this to work you need a friend, and you both need a 2nd controller.
Both players sign a guest in - it must be a guest, not an offline profile (so press Y to sign a guest in, and choose the 'guest' profile from your profiles list).
Player A hosts a Campaign Arcade, select Act 2, Casual.
Player A invites Player B - this will add both Player B and his guest.
Once the game starts - Player B presses 'RT' to take control of his main account - doing this locks him in as Player 3 - the Mauler.
Player A is the host, so he is RAAM, and his guest is Player 2 who is the Elite Theron.
Once you emerge from the E-Hole, start reflecting the COG bullets by holding LT and aiming it like you would a gun.
You will unlock it once you have started planting for the first seeder, as there will be about 12 COGs for you to use. If you down them - do not worry! They either stand-up for you to try again or they bleed-out and it counts.
Once Player B unlocks it, quit out and Player B invites the party to his session.
Repeat the process above for Player A to earn it. Takes under 10mins and works perfectly!